59 



in '''he open sea, away froffi coasts, increasing in number doTvn to a 

 certain depth, while many of them at least are kill-d. bv strong sun- 

 light. It is certain, too, that bacteria ar^^ abundant in many of 

 the muds in moderate and in considerablf^ depths. Do these serve the 

 whole category of mud-eaters as food on the sloped of the continents, 

 and should bacteria be regarded as the primordial meat supply of 

 that celt of the ocean; or is their role in this respect important 

 only locally? We know nothing of their relative abundance at great 

 depths, or in the abyssal mud, except that there, too, they ^xist; 

 but it is here that th- problem assum'-^-s the greatest importance, 

 because of the absence of plants. 



If we trace the food chain back anoth-r link, we face a problem 

 far more signifioant than this simple one of bacteria as prey for 

 animals, namely, their relationship to the circul'ition of nitrogen 

 through its organic and inorganio phases in the sea. 'Yhile the sea 

 water has been found n:'arly saturated with nitrogen gas, none of the 

 ordinary marine plants, so far as we know (and certainly none of the 

 marine animals) can use nitrogen in this elem-'ntal form, though 

 every one of them requires nitrogenous nutriment. For the animals, 

 this food comes in the long run from the plants; and so far as we 

 know all marine plants (except certain bacteria) depend for their 

 existence on the presence of certain salts of nitrogen (chiefly 

 nitrates) in solution in the sea water. 



It has long been known that in the soil certain bacteria are 

 able to assimilate atmospheric nitrog-n, and to fix it in compounds 

 usabl^ by ordinary plants, if they are supplied with other non- 

 nitrogenous sources of energy. These same kinds of bacteria have 

 also been found widespread in the sea in shoal water, at localities 

 as far apart as the Baltic, the North Sea, and the Indian Ocean; , 

 also in the Plankton where the organic carbon going into solution in 

 the water from the break-down of the bodies of defunct animals and 

 plants supplies them with the chemical energy that they require to 

 carry on nitrogen fixation. In f?ct, we h::-.ve experimentgl evidence 

 that they do fix the nitrogen gas with which sea water is saturated, 

 just as they do the atmospheric nitrosen in the soil, on land.^ So 

 far as it goes, their conversion of nitrogen to nitrates must beof 

 direct importance in marine economy, by making available- for marine 

 plants this gaseous source of nitrogen. But we have yet to learn 

 how to assay, in terms of marine economy, the frequency with which 

 such bacteria have been found associated with se3 weeds in shoal 

 watprs (where the concentrations of life ar^- the greatest), for we 

 havp no direct information as to the scale on which they actually 

 operate in the sea. In fact, there is no general agreement as yet 

 as to the actual importance of such of the nitrogen fixers as live 

 free in the soil on land; and this is the group to which belong the 

 marine nitrogen fixers so far known. Neither do we know whether 

 there is anything in the sea comparable to the symbiosis oetween 

 other nitros-en fixers and leguminous plants that exist on land. 

 Solution of'the 2-eneral problem of nitrogen fixation by bacteria 

 in the sea would-be one of the great gifts that marine bacteriolo- 

 gists could offer to Oceanic Biology. 



Opposed to these nitroeen-f ixing bacteria, are the denitri- 

 fying bacteria, that (so far as they operate at all in the sea) tend 

 to reduce the concentration of nitrates (i.e. of available plant 

 food) held in solution in the water, by breaking these down to 

 nitrite, am.raonia, or even to nitrogen and its oxides, and so putting 



